Traveling wave tube demodulator



Oct. 25, 1960 D. E. GEORGE TRAVELING WAVE TUBE DEMODULATOR Filed Sept. 12, 1958 I L W um ON Pan-E um O n O NM an 2 #3 u u Mn F 206mm mTzoEum fl .[Eu 4 x0558: simm 205mm 25w zomhumJw INVEN TOR. DAN/EL E. GEORGE A TTOR/VEY Un t d ates P 1 '.O".

TRAVELING WAVE TUBE DEMODULATOR Daniel E. George, Brooklyn, N.Y., assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed Sept. 12, 1958, Ser. No. 760,782

.5 Claims. 01. 328-227) My invention relates to improved traveling wave tubes.

In the conventional tarveling wave tube, an electron beam is passed through a microwave structure, such as a helical wave guide, in a direction parallel to the axis of the guide and is thereafter collected at a beam collection electrode. A microwave signal whose frequency falls within a given frequency band is supplied at the end of the helix adjacent the electron gun and appears in amplified form at the other end of the helix. The amplification is obtained by adjusting the velocity of signal propagation along the waveguide in an axial direction to be approximately equal to the velocity of the beam. Due to the electromagnetic interaction between the guide and the beam, power is transferred from the beam to the electromagnetic field produced by the guide to provide the desired amplification. This conventional structure can be modified in such manner that the tube can be used as an oscillator or as a limiter.

Recent advances in the microwave art have made it desirable to utilize a traveling wave tube as a microwave demodulator.

Accordingly it is an object of the present invention to produce a traveling wave tube type of structure which will function as a broad band microwave demodulator.

Another object is to provide an improved traveling wave tube structure capable of producing an output current or potential proportional to the amplitude of an'RF signal applied to the input of the tube.

These and other objects of my invention will either be explained or will become apparent hereinafter.

In accordance with the principles of my invention I provide a traveling Wave type tube including an electron gun for producing a beam of electrons. At progressively increasing distances from the gun and disposed in the path of the beam, I provide first a beam modulator, next a beam retarding region which includes a beam retarding electrode, and finally a beam collector electrode. An output impedance is also provided and is coupled to either the beam collector electrode or the beam retarding electrode.

When an incoming RF signal of variable frequency and varying amplitude is applied to the input of the beam modulator, an output potential is developed across the output impedance which, within the band pass of the beam modulator, is proportional to the amplitude of the input signal and independent of its frequency. Thus, a traveling wave tube demodulator is produced. 7

My invention will now be described in detail with reference to the accompanying drawings wherein Fig. 1 illustrates one embodiment of the invention; and

Fig. 2' shows a portion of the device shown in Fig. 1 modified to illustrate another embodiment of the invention.

A helix or slow waveguide 16 is supportedwithin a piece of hollow glass tubing 18 and positioned adjacent the gun 12 in the path of the beam 14. This helical waveguide comprises a beam modulator region 19.

An RF signal is applied to the inputend of the helix through a short coaxial connector 20. A tapered metallic coating 22 is deposited on the outside of the glass tube 18 at the input or first end of thehelix 16 to provide matched coupling between the coaxial connector 20 and thehelix. At the second end of the helix, there is pro-' vided an absorptive attenuator 24, also in the form of a Referring now to Fig. 1, there is shown a traveling g '14 and directingit toward a beam collector at the other tapered metallic coating, but deposited on the inner surface of the glass tubing 18 and connected to the helix. The purpose of the attenuator is to absorb the RF energy on the helix in such manner as to prevent undesirable reflections.

A potential V which is positive with respect to the cathode of the gun 12 is applied to the helix 16 from a. potential source 26. This potential, which may be, for example, of the order of 700 volts, determines the axial velocity of the beam as it passes through the helix. The electron beam is compressed and thus confined within the helix by a magnetic field produced by a coil 28 disposed about the glass envelope 10 and the helix 16.

At a point further along the path of the beam from the attenuator 24, there is provided a planar circular electrode 30 having a central aperture 32. This electrode is connected to the same point of potential as the second. end of the helix 16 and the attenuator 24. A unipotential drift space or region 33 is thus created between the electrode 30 and the second end of the helix, through which the beam electrons are free to drift along without being accelerated, decelerated or deflected. As will become apparent hereinafter, the drift region can be omitted (by eliminating the planar electrode 30) and theelectron beam will then pass directly from the beam modulator region 19 into the retarding region, now to be described.

At points sequentially further along the path of the beam from the planar circular electrode .30, there is provided first, a planar circular electrode '34, similar to the electrode 30, and having a central aperture 36, and secondly, a generally cup-shaped beam retarding electrode 38. The retarding electrode is made slightly negative with respect to the cathode of the gun 12, by a source of potential 37 of the order of approximately 2 volts. The potential difference between the planar electrode 30 and the retarding electrode 38 produces a velocity jump region or beam retarding field region 39 between the planar electrode 30 and the retarding electrode.

As the beam passes through the retarding field region, there is a tendency for the beam to spread, thus reducing the number of electrons that can pass through the aperture 44 in the retarding electrode 38. This is overcome by providing :a conical shaped bottom 42 on the cupshaped retarding electrode and by providing the planar electrode 34 with a positive potential of the order of 50 volts with respect to the cathode of the gun 12. The edges 40 of the cup are pounded to prevent undesirable field emission.

A collector elect-rode 46 is disposed in the path of the electron beam and displaced a short distance from the retarding electrode 38. The collector is connected to asource of potential of approximately 50 volts, positive with respect to the cathode of the gun '12, through an output potential developing impedance, such as, for example, a resistor 48.

As indicated above, my device can be operated satisfacftorily without the presence of the drift region 33 between the :beam modulator and the retarding field. So constructed, my device operates in the following manner;

Patented Oct. 25, 1960,

The electron beam produced by the gun 12 is projected into the region within the helix or beam modulator 16 toward the collector 46. The beam enters the beam modulator region 19 having a certain axial velocity v depending upon the value of the voltage V applied to the helix.

An incoming signal of variable frequency and varying amplitude is applied to the beam modulator 16 through the coaxial connector 20. This signal produces an electro-magnetic field in the region of the beam modulator, which interacts with the electron beam 14. This results in a modulation of the beam whereby the electrons are bunched in a well known manner; i.e. a. velocity distribution of the electrons along the path of the beam is produced in accordance bothwith the frequency and the amplitude of the incoming signal. Accordingly, at any given instant of time the electrons at different points of the beam have different velocities.

The beam next enters the retarding field region 39. The retarding field has the effect of varying the velocity distribution of electrons along the path of the beam in such manner that it is no longer a function of both the amplitude and frequency of the incoming signal, but is a function only of the amplitude thereof. However, the bunching reflect remains, so that as before the electrons at difierent points along the beam have diiferent velocities.

As the beam passes through the retarding field, its energy and velocity are gradually reduced until at a point 45 near the aperture 44, it has an average velocity close to zero. Therefore, since the beam comprises electrons having different velocities, some now have very low velocities, close to zero, and others have higher velocities.

The low velocity electrons have insufiicient energy to pass through the aperture 44 in the negatively charged retarding electrode 38; these electrons therefore do not reach the collector electrode 46, but are attracted to the positively charged electrodes 30 land 34 and the attenuator 24. However, the higher velocity electrons have sufiicient energy to pass through the aperture 44 and hence do reach the collector 46. As these higher velocity electrons pass through the collector circuit, they develop an output potential across the terminals 50 of the resistor 48. Since the electron velocity distribution along the path of the beam is, at the end point 45 of the retarding field, a function only of the amplitude of the signal input to the modulator, the output potential across the resistor 48 is a function only of that amplitude. I have thus produced a traveling wave tube demodulator which provides an output signal which varies in accordance with the incoming signal amplitude and which is independent of the trequency thereof.

As previously indicated, the operation of my device, constructed without the beam drift region 33, is very satisfactory. I have found however that an output signal of increased amplitude is obtainable firom said device When a drift region is provided between the beam modulator region 19 and the retarding field region 39.

Fig. 2 illustrates an alternative method of obtaining the output signal. In this embodiment the output resistor 48' is connected in the circuit of the retarding field electrode 38 instead of the collector 46, and the collector is connected directly to its source of positive potential of approximately 50 volts. The electron flow in the circuit of the retarding field electrode is, as in the case of the collector electrode, also a function of the input signal amplitude. Therefore an output potential be developed across the resistor 48' which is, as in Fig. 1 a function only of the amplitude of the input signal to the beam modulator.

What is claimed is:

1. A travelling wave tube comprising an electron gun, a helical coil, la first apertured conductive disc, a second apertured conductive disc, a cylindrical electrode having an-intcrnal cone-shaped base, said base being apertured at its apex, and a cylindrical collector electrode, all of said elements being arranged, in the order named, along a common axis so that electrons emitted from said gun pass through said coil and through the apertures in said discs and apertured cone-shaped base to said collector electrode, the apertured apex of said cone being arranged closer to said collector than to the base of said cone.

2. A travelling wave tube comprising an electron gun, a helical coil, a first conductive apertured disc, a second conductive apertured disc, a cylindrical electrode having an internal cone-shaped base, said base being apertured at its apex, and a cylindrical collector electrode, all of said elements being arranged, in the order named, along a common axis, said first mentioned apertured disc being directly connected to said helical Wave coil.

3. A travelling wave tube comprising an electron gun, a helical coil, a first apertured disc of conductive material, a second apertured disc of conductive material, a cylindrical electrode having an internal cone-shaped base, saidbase being apertured at its apex, a cylindrical collector electrode, all of said elements being arranged, in the order named, along a common axis, an absorptive attenuator connected to the end of said coil remote from said gun, and means directly connecting said first apertured disc to said absorptive attenuator.

4. Demodulating apparatus comprising a tube having, along a common axis and in the order named, a cathode, a wave coil, a terminating attenuator, a first apertured disc, 21 second apertured disc, a cylindrical retarding electrode having a centrally pierced conical-shaped bottom, and, a collector electrode; means applying short electrical waves to the end of said wave coil nearest said cathode, the other end of said coil being connected to said attenuator; means connecting said first apertured. disc directly to said attenuator; means for applying apositive potential to said wave coil, attenuator and first apertured disc with respect to said cathode; means applying a positive potential to said second apertured disc with respect to said'cathode; means applying a negative potential to said cylindrical retarding electrode with respect to said cathode; means applying a positive potential to said collector electrode with respect to said cathode and an output impedance, across which demodulated voltages are set up, connected between said collector electrode and said cathode.

5. Demodulating apparatus comprising a tube having, along a common axis and in the order named, a cathode, a slow waveguide, a terminating attenuator, a first apertured disc, a second apertured disc, a cylindrical retarding electrode having a centrally pierced conical-shaped bottom, and, a collector electrode; means applying short electrical waves to the end of said slow waveguide nearest said cathode, the other end of said guide being connected to said attenuator; means connecting said first apertured disc directly to said attenuator; means for applying a positive potential to said waveguide, attenuator and first apertured disc with-respect to said cathode; means applying a positive potential to said second apertured disc with respect to said cathode; means applying a negative potential to said cylindrical retarding electrode with respect to said cathode; means applying a positive potential to said collector electrode with respect to said cathode; and an output resistor, across which demodulated voltages are set up, connected between said collector electrode and said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,222,898 Fraenckel Nov. 26, 1940 2,240,183 Hahn Apr. 29, 1941 2,284,733 Haeff- June 2, 1942 2,300,052 Lindenblad Oct. 27, 1942 2,325,865 Litton Aug. 3, 1943 2,414,843 Varian Ian. 28, 1947 2,853,641 Webber Sept. 23, 1958 

